The human macrophage, an important component of the immune system, in large part functions to detect and damage foreign cells. The interaction between human macrophages and human erythrocyte target cells serves as a study model for immune macrophage-lymphoblastoid cell interaction. Human erythrocytes coated with IgG, C3 or concanavalin A have been observed to bind to homologous marophages and become damaged over time. The role of antigen in this recognition as well as the pharmacologic agents, corticosteroids, levamisole, and BGG are being studied. Macrophage-lymphoblastoid cell interaction is being examined as a model for macrophage tumor cell interaction. A spontaneous interaction has been observed and characterized. The capacity of complement to augment this interaction is being studied, using tissue as well as circulating macrophages. Delineation of the mechanisms involved in macrophage recognition and recognition of altered cell surfaces should provide insight into new approaches for harnessing the macrophage's ability to detect tumor cells and modify their viability.